The diagnosis of dementia of the Alzheimer type (DAT) is uncertain in life partly because of difficulties in discriminating the physiological and cognitive changes accompanying normal aging from those associated with the early stages of DAT. Recently, however, alterations in patterns of regional cerebral glucose metabolism have been reported to be present in early DAT. These metabolic changes may reflect compensatory brain mechanisms that are operative in both normal aging and in early DAT, but which break down as the disease progresses. The strength of such compensatory processes may depend upon the efficiency of attentional mechanisms, particularly in response to increased information-processing demands. A neuropsychological model is proposed that integrates these findings with the results of preliminary studies indicating that simple cognitive operations are preserved at greater "attentional cost" in early DAT. The model proposes that inefficient attentional mechanisms represent an early functional impairment in DAT and are underlying factors in the memory and cognitive deficits of DAT. Furthermore, the magnitude of attention-related compensation mechanisms in the early stages of DAT may predict the rate of subsequent cognitive decline. The major aim of the proposed research is to evaluate this model in a series of six studies in four groups of subjects: young adults, older adults, older adults rigorously screened for health status, and DAT patients. Two methods for manipulating attention- -reaction time to a "probe" stimulus, and covert shifts of attention--will be used to evaluate the attentional demands of cognitive operations in aging and early DAT. Studies 1 and 3 will examine two aspects of attention--(i) the attentional demands of simple cognitive operations and (ii) the switching of attention from one spatial location to another--in all four groups of subjects. Studies 2 and 4 will examine more complex features of these attentional mechanisms in young and older adults. Study 5 will examine interrelationships between the attentional mechanisms investigated in Studies 1 and 3 and cerebral metabolic rates for glucose as assessed by 18-FDG positron emission tomography (PET). In study 6 healthy elderly and DAT subjects will be given yearly follow-up evaluations on PET and the Study 1 attention tests for a period of four years. The proposed research will identify changes in attentional and physiological function in the early stages of DAT which have not been investigated extensively, and also advance understanding of attentional changes accompanying normal aging. The results should contribute to the development of cognitive and physiological markers for early diagnosis of DAT.